Voltage balancing circuit for series connected capacitor banks or voltage cells and variable frequency drive for driving an electric motor or power converter for supplying a load with a voltage balancing circuit

ABSTRACT

A voltage balancing circuit for capacitor banks or voltage cells connected in series. The circuit includes a DC-link with at least two capacitors or voltage cells connected in series and at least two emitter follower balancing circuits connected in parallel. At least one emitter resistor is provided between the emitter of each emitter follower balancing circuit and the mid-point of the DC-link. The gate emitter voltage applied to each emitter follower balancing circuit may be equal to its common gate voltage minus the voltage drop on the corresponding emitter resistor. The invention is also directed at a variable frequency drive for driving an electric motor or a power converter, having a corresponding voltage balancing circuit.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims foreign priority benefits under 35 U.S.C. § 119from German Patent Application No. 102022103410.0, filed Feb. 14, 2022,the content of which is hereby incorporated by reference in itsentirety.

TECHNICAL FIELD

The present invention pertains to a voltage balancing circuit forcapacitor banks or voltage cells connected in series. The circuitcomprises a DC-link with at least two capacitors or voltage cellsconnected in series and at least two emitter follower balancing circuitsconnected in parallel. At least one emitter resistor is provided betweenthe emitter of each emitter follower balancing circuit and the mid-pointof the DC-link. The gate emitter voltage applied to each emitterfollower balancing circuit may be equal to its common gate voltage minusthe voltage drop on the corresponding emitter resistor. The invention isalso directed at a variable frequency drive for driving an electricmotor or a power converter, comprising a corresponding voltage balancingcircuit.

BACKGROUND

Voltage balancing circuits actively regulate the voltage balance betweencapacitors or voltage cells connected in series such as battery cells. Aproblem of known voltage balancing circuits is that the maximumimbalanced current compensated by the circuit is practically limited bythe given architecture of the circuit.

SUMMARY

The aim of the invention is to provide an improved voltage balancingcircuit and an improved variable frequency drive or power converter,which offer a simple way of adapting the device to various amounts ofimbalanced currents.

This aim is achieved by the voltage balancing circuit according to claim1 and the variable frequency drive or power converter according to claim10. Preferable embodiments are subject of the dependent claims.

According to claim 1, a voltage balancing circuit for capacitor banks orvoltage cells connected in series is provided. It comprises a DC-linkwith at least two capacitors or voltage cells connected in series and atleast two emitter follower balancing circuits connected in parallel.According to the invention, at least one emitter resistor is providedbetween the emitter of each emitter follower balancing circuit and themid-point of the DC-link. The gate emitter voltage applied to eachemitter follower balancing circuit may be equal to its common gatevoltage minus the voltage drop on the corresponding emitter resistor.

This circuit is scalable in the sense that higher imbalance currents canbe compensated by placing one or more circuits in parallel. It offerslower cost, better performance and practically unlimited scalabilitycompared to the state of the art. The present invention offers asolution for paralleling unlimited or at least a high number of emitterfollower balancing circuits and ensuring good sharing between theparallel circuits and in particular between the parallel elements suchas IGBT-s or MOSFET-s, despite parameter differences in the transistorand resistor components used. At the same time, it provides a veryaccurate balancing function between the in series connected capacitorsor voltage cells. The lower count of components and the simplicity ofthe presently described design contribute towards its enhancedreliability. The presently described circuit can be used in allapplications that require in series connections of capacitors or voltagecells like batteries. It provides good balancing at lower costs comparedto known solutions.

According to a preferred embodiment of the invention, three, four ormore parallel emitter follower balancing circuits are provided. It istherefore easy to scale the circuit by adding some relatively cheapcomponents to it. The increased number of emitter follower balancingcircuits may be chosen such that a desired higher imbalance current canbe compensated.

According to another preferred embodiment of the invention, at least onecommon resistor connects the emitter resistors to the negative DC-linkside.

According to another preferred embodiment of the invention, the parallelemitter follower balancing circuits comprise IGBTs and/or MOSFETs. Theemitter follower is not limited to IGBTs or MOSFETs but can be any typeof transistors, say bi-polar, FET, Darlington. Uni-junction transistorsmay be excluded by the invention.

According to another preferred embodiment of the invention, thecapacitor banks or voltage cells comprise battery cells and/or fuelcells and/or capacitors. The term capacitor banks may be thereforeunderstood in a broad sense and may comprise components other thancapacitors.

According to a particularly preferred embodiment of the invention, thebattery cells and/or fuel cells and/or capacitors are arranged inparallel and/or in series to each other. Accordingly, a considerablenumber of said components may be provided for enabling the aboveoutlined arrangement.

According to a preferred embodiment of the invention, at least threecapacitors or voltage cells are connected in series to each other and/orat least two capacitors or voltage cells are connected in parallel to atleast two other capacitors or voltage cells. The number and arrangementof the capacitors or voltage cells may be selected to suit high voltagesoccurring at the DC-link. Thus, a higher number of relatively cheap andsmall components can be used for high voltage applications, which wouldotherwise require more expensive and larger components.

According to another preferred embodiment of the invention, the voltagebalancing circuit is integrated in a variable frequency drive fordriving an electric motor or in a power converter to supply a load.

The invention is also directed at a variable frequency drive for drivingan electric motor or power converter for supplying a load, comprising avoltage balancing circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

Other details and advantages of the invention will be described withreference to the figures. The figures show:

FIG. 1 a : balancing of the in series capacitors voltage with inparallel resistors according to the state of the art;

FIG. 1 b : active balancing circuit based on emitter follower topologyaccording to the state of the art;

FIG. 1 c : paralleling of active balancing circuits according to thestate of the art;

FIG. 2 : current sharing self-balancing circuit for in series capacitorsvoltage active balancing;

FIG. 3 : simulation results of presently described circuit;

FIG. 4 : serial emitter follower balancing circuits having serialcoupled DC link capacitors; and

FIG. 5 : parallel emitter follower balancing circuits having parallelcoupled DC link capacitors.

DETAILED DESCRIPTION

Known variable frequency drives (VFD) are based on a voltage-controlledinverter (VCI) topology. This topology requires a stable DC link voltageto supply the inverter. Therefore, a DC-link capacitor bank is commonlyprovided as an energy storage and a decoupling device between therectifier and the inverter stages.

To achieve the needed capacitance value and the voltage required for theDC bank, several capacitors C1, C2 may be connected in parallel and/orseries, as shown in FIG. 1 a . The in series connection of capacitorsC1, C2 is used to achieve the desired DC-link voltage when usingcapacitors C1, C2 of a lower rated voltage either because higher voltagecapacitors C1, C2 do not exist or because the cost and/or performancecan be improved with lower voltage rated capacitors C1, C2. Whicheverthe reason, the series connection of capacitors C1, C2 is a very commonpractice not only in VFD but also in many other applications such asbattery cells and fuel cells or power converters.

It is well known that connecting two or more capacitors C1, C2 in seriescan lead to an uneven voltage sharing between the individual capacitorsC1, C2, even if the capacitors C1, C2 are near identical parts andprovided by the same manufacturer. Because of the manufacturingtolerances of the materials and capacitor construction or aging,differences between individual capacitors C1, C2 in capacitance, leakagecurrent etc. always exist. These differences lead to uneven voltagesharing between series connected capacitors C1, C2. As a result, thevoltage applied across one capacitor C1, C2 may even exceed its ratedvoltage and consequently lead to a premature failure of the capacitorC1, C2.

To ensure a balanced sharing between the in series connected capacitorsC1, C2, it is common to connect high power resistors R1, R2 in parallelwith each capacitor C1, C2. The values of these resistors R1, R2 can becalculated based on the leakage current of the individual capacitors C1,C2 and the maximum tolerance of the leakage current of the capacitorsC1, C2. The parallel resistors R1, R2 are a simple solution of theproblem, but they turn out to be relatively large in terms of powerdissipation. They are not very cost effective in the overall picture ofthe application. A typical circuit diagram of this known solution isshown in FIG. 1 a , where the voltage of in series capacitors C1, C2 isbalanced with in parallel resistors R1, R2.

An alternative way to ensure the voltage balance between the capacitorsconnected in series C1, C2 is shown in FIG. 1 b . Here, an activebalancing circuit is shown, which is based on emitter follower topology.This design differs from the previously described passive solution ofthe parallel resistors R1, R2. It represents an active circuit as it isbased on active components, i.e. transistors, which adjust their currentand/or resistance proportionally to the leakage current differencesbetween the capacitors C1, C2.

When designed properly, active balancing solutions can provide lowerpower dissipation, better balancing, and an overall more cost-effectivesolution. Several of these active balancing circuits are known to beused for balancing the voltage of capacitors C1, C2 connected in seriesas well as other applications, like battery cells, fuel cells etc.connected in series.

Active balancing circuits are known to be based on well-known emitterfollower circuits. These circuits may use a high voltage, low currentIGBT or Insulated Gate Bipolar Transistor as the main active balancingcomponent. Additionally, they use several resistors R1, Rd1, Rg1, Rg2,which are much smaller and less costly than a passive balancing parallelresistor circuit.

In the circuit of FIG. 1 b , reference sign FET1 refers to a transistorsuch as an IGBT or a MOSFET. The transistor FET1 operates in the activeregion and therefore the power dissipation in the transistor FET1 canincrease to levels that the device package cannot support.

In cases where relatively large capacitor current leakage imbalancesneed to be compensated by the active balancing circuit, two or moreactive balancing circuits may be needed. Such a parallel activebalancing circuit is shown in the circuit diagram of FIG. 1 c.

Here, the current sharing between the two parallel transistors FET1,FET2 needs to be balanced. It is known that transistors such as IGBT andMOSFET devices are typically designed for high voltage and low currentranges. They are primarily designed for switching operations. The devicecurrent is controlled by its gate voltage. When the gate voltage isbelow the threshold voltage, the current through the device is verysmall and it takes very little gate voltage above the threshold to placethe device in a low resistance state. At the threshold gate voltagelevel an almost immediate change in the device from off to on occurs.However, for an active balancing circuit, a smoothly controllable“resistor” behaviour of transistors such as IGBT or MOSFET is desiredand not a switch behaviour between fully off and fully on states.

When transistors are utilized in the active operation region, very smalldifferences in their threshold gate voltages in the region of millivoltsmay exist between two otherwise near identical transistors. Thesedifferences may cause one transistor to turn on faster and carry almostall the current and the other parallel transistor to carry almost nocurrent. The branch that has the lower threshold voltage transistor willcarry more current and dissipate much higher power than the branch withhigher threshold gate voltage transistor.

This makes the utilization of two or more circuits in parallelinefficient. The present invention provides a circuit that overcomes thecurrent sharing problem between parallel active balancing circuits andimproves the current sharing balance between parallel circuits.

FIG. 2 shows a current sharing self-balancing circuit for seriescapacitors voltage active balancing according to the present invention.

The circuit comprises a DC-link with at least two capacitors or voltagecells C1, C2 connected in series. A mid-point Vdcm of the DC-link issituated between the two capacitors C1, C2. Although reference will bemade primarily to capacitors, voltage cells may be used alternatively.The circuit further comprises at least two emitter follower balancingcircuits FET1, FET2 connected in parallel with at least one emitterresistor Rs1, Rs2 provided between the emitter of each emitter followerbalancing circuit FET1, FET2 and the mid-point Vdcm of the DC-link. Thevoltages and currents across said emitter resistors Rs1, Rs2 areindicated as Vfbk1, Vfbk2 and Is1, Is2, respectively. To evenly balancethe voltage over C1, C2, the value of Rg1 is set to or close to thevalue of Rg2, meaning Rg1=Rg2. The values of Rg1 and Rg2 may differslightly. In particular, Rg1 may be smaller than Rg2 to accomodate forthe gate voltage of the MOSFET or IGBT. The exact relationship may bedescribed by the following expression:

Rg1=(Vdc/(Vgeth+Vdc/2)−1)*Rg2

where Vdc is the DC link voltage and Vgeth is the nominal gate emitterthreshold voltage of the MOSFET.

The gate emitter voltage applied to each emitter follower balancingcircuit FET1, FET2 may be equal to its common gate voltage minus thevoltage drop on the corresponding emitter resistor Rs1, Rs2.

The basic architecture of the circuit is chosen such that three, four ormore emitter follower balancing circuits FET1, FET2 connected inparallel can be provided easily. However, they are not shown in thefigures. If more than two emitter follower balancing circuits FET1, FET2connected in parallel are used, they may all be integrated within theremainder of the voltage balancing circuit as is presently describedwith reference to the first two emitter follower balancing circuitsFET1, FET2. For example, each of the plurality of emitter followerbalancing circuits FET1, FET2 may be connected to the mid-point Vdcm ofthe DC-link via a corresponding emitter resistor Rs1, Rs2.

In the embodiment of FIG. 2 , the two emitter resistors Rs1, Rs2 are notonly connected to the mid-point Vdcm of the DC-link but also to thenegative DC-link side Vdc-via at least one common resistor R1. Again, ifmore than two emitter resistors Rs1, Rs2 are used in the voltagebalancing circuit, they all may be connected to the negative DC-linkside Vdc-via the at least one common resistor R1.

The parallel emitter follower balancing circuits FET1, FET2 may compriseIGBTs and/or MOSFETs and in particular identical IGBTs and/or MOSFETs.The capacitor banks or voltage cells C1, C2 may comprise components suchas battery cells and/or fuel cells and/or capacitors, which may beprovided in identical pairs and in series and/or parallel connectionwith respect to each other. Clearly, the series and/or parallelconnection of the components may be realized with a substantial numberof the named components, which may at the same time be arranged in nearidentical pairs of series connected components.

The presently described voltage balancing circuit may be integrated in avariable frequency drive. The invention is also directed at acorresponding variable frequency drive for driving an electric motor.The drive comprises a voltage balancing circuit as presently described.

According to FIG. 2 , the invention is realized by placing smallresistors Rs1, Rs2 between the emitter of each emitter followerbalancing circuits FET1, FET2 and the midpoint of the DC-link Vdcm. Theemitter follower balancing circuits FET1, FET2 may be presently referredto as transistors FET1, FET2 for simplicity's sake.

These resistors Rs1, Rs2 carry the emitter current of each transistorFET1, FET2 and the gate emitter voltage applied to each transistor FET1,FET2 is equal to the common gate voltage minus the voltage drop on theresistors Rs1, Rs2. If the transistor FET1 has a lower threshold voltagethan the second transistor FET2, the common gate emitter voltage appliedto both transistors FET1, FET2, i.e. when no emitter resistors Rs1, Rs2were mounted in FIG. 2 , will favour the first transistor FET1 to turnon faster than the second transistor FET2. Consequently, the firsttransistor FET1 will be carrying the majority of the current.

When the emitter resistors Rs1, Rs2 are mounted as shown in FIG. 2 ,then the larger current flowing through the first transistor FET1increases also the voltage drop on the resistor Rs1 placed in serieswith its emitter. Therefore, the gate emitter voltage applied to thefirst transistor FET1 is lowered on the one side. In effect, theresistance is increased and the current lowered.

On the other side, the gate emitter voltage applied to the secondtransistor FET2 is larger than that applied to the first transistorFET1, since the current through the second transistor FET2 and thereforethe voltage drop in its emitter resistor Rs2 are small. This way, thefirst transistor FET1, that has a smaller threshold voltage, also has asmaller voltage applied to its gate emitter than the second transistorFET2.

The circuit of FIG. 2 balances the current between parallel transistorsFET1, FET2. The resistors Rs1, Rs2 placed at the emitter of eachparallel transistor FET1, FET2 act as a negative closed loop feedback tobalance their current sharing. Larger values for the feedback resistorswill ensure better balancing but increasing these resistor values willalso increase the power loss on them. More importantly, the error inbalancing the voltage between series connected capacitors C1, C2 isincreased. This is because the emitters of the transistors FET1, FET2are not connected directly to the mid-point Vdcm of the DC-link, butthrough resistors Rs1 and Rs2.

FIG. 3 provides simulation results of the circuit in FIG. 2 , whereinthe series connected capacitors C1, C2 have different leakage currentsin parallel with each capacitor C1, C2. As transistors, two MOSFETs havebeen selected for the simulation, that have slightly different thresholdvoltages, 3.98V and 4.02V. In the simulation, resistors Rs1, Rs2 arevaried simultaneously from 0-400 Ohm.

In FIG. 3 , the bottom plot shows the middle point between two seriesconnected capacitors C1, C2. The active balancing circuit works to keepthis voltage constant at half DC-link voltage despite the two capacitorC1, C2 branches having leakage currents that differ by a factor of morethan 3 from each other.

The second plot from the bottom shows the currents that flow in each ofthe parallel branches. The top line refers to the current through Rd2and the bottom line to the current through Rd1. At Rs1=Rs2=0 or somevery small value, the currents differ from each other. As theresistances Rs1 and Rs2 are increased, the currents start to mergetoward the same value, in effect sharing the total current well witheach other.

The plot in the middle of FIG. 3 shows the Vge voltage applied on eachtransistor FET1, FET2. It confirms that the transistor FET1, FET2 withlower threshold voltage has also lower Vge applied when Rs2 and Rs1 havesome non-zero value.

The two upper plots in FIG. 3 show the power dissipation in transistorsFET1, FET2 and resistors Rd1 and Rd2. Clearly, as the resistance valueof resistors Rs2 and Rs1 is increased, the power dissipations start tomerge toward each other, improving the balance between the two parallelbranches of the active circuit.

FIG. 4 shows the serial emitter follower balancing circuits FET1, FET2having serial coupled DC link capacitors C1, C2, C3. The serial couplingof the DC link capacitors C1, C2, C3 may comprise three or morecapacitors to withstand high DC-link voltage of say 750 Vdc and wherecapacitor max voltage rating of a single capacitor cannot meet therequirement or may just not be feasible to use in terms of size, price,performance, and/or availability.

FIG. 5 shows the parallel emitter follower balancing circuits FET1, FET2having parallel coupled DC link capacitors C1, C2, C3, C4. The parallelcoupling of DC-link capacitors may comprise two or more DC-linkcapacitors coupled in parallel with C1, C2 in order to get highercapacitance than C1, C2 can offer on their own. The price, size,performance, and/or availability may be reasons for coupling moreDC-link capacitors in parallel.

While the present disclosure has been illustrated and described withrespect to a particular embodiment thereof, it should be appreciated bythose of ordinary skill in the art that various modifications to thisdisclosure may be made without departing from the spirit and scope ofthe present disclosure.

What is claimed is:
 1. A voltage balancing circuit for capacitor banks or voltage cells connected in series, comprising a DC-link with at least two capacitors or voltage cells connected in series and at least two emitter follower balancing circuits connected in parallel, wherein at least one emitter resistor is provided between the emitter of each emitter follower balancing circuit and the mid-point of the DC-link.
 2. The voltage balancing circuit according to claim 1, wherein three, four or more parallel emitter follower balancing circuits are provided.
 3. The voltage balancing circuit according to claim 1, wherein at least one common resistor connects the emitter resistors to the negative DC-link side.
 4. The voltage balancing circuit according to claim 1, wherein the parallel emitter follower balancing circuits comprise IGBTs and/or MOSFETs.
 5. The voltage balancing circuit according to claim 1, wherein the capacitor banks or voltage cells comprise battery cells and/or fuel cells and/or capacitors.
 6. The voltage balancing circuit according to claim 5, wherein the battery cells and/or fuel cells and/or capacitors are arranged in parallel and/or in series to each other.
 7. The voltage balancing circuit according to claim 1, wherein at least three capacitors or voltage cells are connected in series to each other and/or that at least two capacitors or voltage cells are connected in parallel to at least two other capacitors or voltage cells.
 8. The voltage balancing circuit according to claim 1, wherein the voltage balancing circuit is integrated in a variable frequency drive for driving an electric motor.
 9. The voltage balancing circuit according to claim 1, wherein the voltage balancing circuit is integrated in a power converter to supply a load.
 10. A variable frequency drive for driving an electric motor or power converter for supplying a load, comprising the voltage balancing circuit according to claim
 1. 11. The voltage balancing circuit according to claim 2, wherein at least one common resistor connects the emitter resistors to the negative DC-link side.
 12. The voltage balancing circuit according to claim 2, wherein the parallel emitter follower balancing circuits comprise IGBTs and/or MOSFETs.
 13. The voltage balancing circuit according to claim 3, wherein the parallel emitter follower balancing circuits comprise IGBTs and/or MOSFETs.
 14. The voltage balancing circuit according to claim 2, wherein the capacitor banks or voltage cells comprise battery cells and/or fuel cells and/or capacitors.
 15. The voltage balancing circuit according to claim 3, wherein the capacitor banks or voltage cells comprise battery cells and/or fuel cells and/or capacitors.
 16. The voltage balancing circuit according to claim 4, wherein the capacitor banks or voltage cells comprise battery cells and/or fuel cells and/or capacitors.
 17. The voltage balancing circuit according to claim 2, wherein at least three capacitors or voltage cells are connected in series to each other and/or that at least two capacitors or voltage cells are connected in parallel to at least two other capacitors or voltage cells.
 18. The voltage balancing circuit according to claim 3, wherein at least three capacitors or voltage cells are connected in series to each other and/or that at least two capacitors or voltage cells are connected in parallel to at least two other capacitors or voltage cells.
 19. The voltage balancing circuit according to claim 4, wherein at least three capacitors or voltage cells are connected in series to each other and/or that at least two capacitors or voltage cells are connected in parallel to at least two other capacitors or voltage cells.
 20. The voltage balancing circuit according to claim 5, wherein at least three capacitors or voltage cells are connected in series to each other and/or that at least two capacitors or voltage cells are connected in parallel to at least two other capacitors or voltage cells. 